Rainfall and moisture in Brazil

Brazil is one of the wettest countries in the world, yet the regional variation is large (Fig 1). The country can be divided into five regions, as regards rainfall (1).

The far south (Rio Grande do Sul, south of 24°S) receives rain throughout the year, fairly evenly, although summertime rain is more convective in nature and winter rain more stratiform, mostly around the passage of a cold front.

The northwest (the Amazon basin) also has a fairly uniform rainfall distribution, although it is wetter (2-3 m/a) and at all times the rainfall is largely convective. The dewpoint at Manaus (at 3°S) remains at about 23°C all year. The precipitable water (Section 6.6) in Manaus averages around 50 mm, and peaks around 60 mm in October. The central region (Roraima and Para) and the southern region (Mata Grosso and Rondonia) experience a brief but well-marked dry season, from May through October (Fig 2).

The central interior region (10-23°S) has zenithal rains, as is typical for that latitude. About 75% of the annual rainfall occurs between 21 September and 21 March, and the winter is relatively dry. Over half the annual rain falls in the three months December - February. Again, almost all rain is convectively induced. Brasilia (at 15°S, on the interior Central Plateau) is in the third of these regions. Dewpoint measurements show an abrupt rise from around 10°C to about 17°C during September, and a more gradual decline again from March until May.

Northeast Brazil receives less than 800 mm per year, due to the intrusion of the anticyclone prevailing in the south Atlantic (Fig 12.1). Some inland regions receive less than 400 mm/a. The northern part of the Nordeste (1-6°S) has most of its rain in autumn (March to May), and it is mainly convective. Rainfall amounts vary greatly from year to year. Droughts in the Nordeste are more likely in El Niño years. Unlike the Amazon, the Nordeste has long been developed, and during serious droughts thousands of people move the overpopulated cities of the south, or into the Amazon region. The coastal area from Recife southward (bordering region 5) receives more rain, year-round.

The narrow coastal strip between 6-20°S receives abundant rainfall, in part because of the coastal mountains. Rain falls year-round but there is a maximum in winter, with mostly stratiform rainfall.

Figure 1 shows a 'ridge' of maximum rainfall from northwest to southeast, esp. just south of Brasilia and in Brazil's far south (Parana - Santa Catarina). This is due to the higher topography there (a plateau about 1,000m high), and to the South Atlantic Convergence Zone.

Fig 1. Annual mean rainfall (mm) in Brazil (1).

Fig 2. Percentage of the annual total rainfall which falls in a) December - February, and b) June - August (based on1958-78 and 1985-89 data) (2).

Water vapour content and transport

Dewpoint measurements at Brasilia show that the transition from winter to summer conditions occurs within two or three weeks in October and is due to changes in the pattern of winds. The moisture of the summer rain in central Brazil comes largely from the Amazon basin, which in turn receives its moisture from the equatorial Atlantic (Fig 3). There is a broad, large anticyclonic (i.e. anticlockwise) moisture transport belt between the Andes and the South Atlantic high (Fig 4). There is also an appreciable transfer of moisture southeastwards in transient lows south of about 20ºS, especially in July. The moisture transfer at low latitudes is higher than that in midlatitudes where strong westerlies blow, because the mixing ratio is higher in the tropics.

Fig 3. The vertically integrated total water vapour transport in summer and winter months in South America. Each arrow has a length proportional to the flux, which is expressed in units of kg/m.s. The arrow below each diagram has a length equivalent to 300 of these units.

Fig 4: Mean streamlines (lines with arrows) and isotachs (bold lines, units are m/s) in January and July over South America (1).

The precipitable water over the Amazon Valley is more than 50 kg/m2, equivalent to a depth of 50 mm (Fig 5). These are the highest mean values on Earth; that is the atmosphere over the Amazon typically is moist at all levels, and therefore the lapse rate is nearly moist-adiabatic. The average evaporation rate is 4.5 mm/day, so the average turnover time is 11 days (i.e. 50/4.5). (This resembles times for higher latitudes, where lower temperatures reduce both the evaporation rate and also the precipitable water content of the atmosphere.)